Corrosion inhibitive coatings and sealants, such as disclosed by U.S. Pat. No. 3,730,937 to Boggs and Miller, utilize toxic chromates as the corrosion inhibitor compound. While the corrosion inhibitive, chromate containing polysulfide coatings and sealants proposed by Boggs and Miller prevent exfoliation corrosion of fastener holes as well as faying surface corrosion between adjacent exterior panels to thereby greatly extend the operational life of metallic structures, such as aircraft and the like; there is growing concern with difficulties encountered in the disposal of the toxic chromate containing waste associated with such corrosion inhibitive coatings and sealants.
In order to eliminate the waste disposal problems associated with chromate containing coating and sealant materials, less toxic inhibitor compounds have been investigated such as sodium nitrate, sodium molybdate and sodium metasilicate. However, in order to achieve the same level of corrosion inhibition that is provided by chromate containing coatings and sealants, approximately five times as much of the non-toxic inhibitor compound must be added to the sealant material. Moreover, when formulations containing these non-toxic inhibitor compounds are added to two part polysulfide sealants, the cure rate of the polysulfide sealant material is adversely effected resulting in either an unacceptable acceleration or retardation of the cure. While encapsulation of the inhibitor compounds has been proposed as a solution to the cure problem, it is both an expensive as well as time consuming process.
In addition to corrosion, metallic structures which are cyclically stressed, such as aircraft and the like, suffer from environmentally enhanced fatigue cracking. For example, the fatigue cracking of high strength aluminum in a salt water environment is more than double that experienced in a dry desert-like environment. Environmentally enhanced fatigue cracking is, essentially, a hydrogen embrittlement phenomenon and can be related to the corrosion process. When water reacts with a metal such as aluminum the corrosion products are aluminum hydroxide and hydrogen. In a fatigue cracking situation, the nascent atomic hydrogen migrates to the zones of maximum stress at the crack tip and, by its physical presence, decreases the force required to pull grains apart. Research has shown that the best corrosion inhibitors, such as the chromates, have little effect on the rate of fatigue cracking of metals such as aluminum alloys once a crack has initiated.